In recent years, lithium ion secondary batteries have been incorporated in a large number of mobile phones and laptops, among other electronic appliances. Lithium ion secondary batteries have high energy densities and, to ensure the safety of users and electric appliances, are typically provided with several protective circuits incorporated in the battery pack for over-charging protection and over-discharging protection to interrupt the input and output of the battery pack under predetermined conditions. However, if a positive/negative electrode insulation fitting portion were to be corroded by being wet, there is a possibility that pressure from the interior of the battery might leak and a safety valve might malfunction to cause a fire.